It's best to do an incremental upgrade from old versions in the sequence 2.3 to 2.5, 2.5 to 2.8.5, 2.8.5 to 2.9.2. In theory you should be able to upgrade from 2.0.5 straight to 2.9 but people report more success with the incremental approach to ensure the database gets upgraded properly. Older versions available at http://wordpress.org/download/release-archive/